The present invention relates in general to the control of a sensorless and brushless DC (BLDC) motor. More particularly, the present invention relates to a sensorless BLDC motor operating at PWM (Pulse Width Module) mode.
Generally, BLDC motors use semiconductor drive circuits in DC motors so that commutators and brushes are not needed. Namely, BLDC motors are able to control current using semiconductor switches comprised of inverters, and include an armature with a plurality of windings performing a stator function, a permanent magnet performing a rotor function, and a position detecting portion performing a brush and commutator function.
In order for the motor to function correctly, the flux existing in the stator must always be about 90 electrical degrees in advance of the rotor to continually pull the rotor forward. However, the rotor movement and the flux rotation should never be allowed to get out of synchronization, as the rotor may stop turning, or in any case will become very inefficient. Therefore, to obtain an average electromagnetic torque as high as possible and to optimize the efficiency of the motor, the switching of the windings from one step to another, namely, xe2x80x9celectronically commutationxe2x80x9d, must be controlled in accordance with the actual position of the rotor.
Generally speaking, the BLDC motor above is controlled by detecting the position of the rotor, then applying a current to a stator according to this position. A position detection part is provided to detect the position of the rotor using either a Hall method utilizing a Hall element, an optical method, a high frequency induction method, a high frequency oscillation method, a reed switch method, or a magnetoresistive element method to detect the position of the rotor. However, when using a BLDC motor in a compressor, high temperatures and high pressures caused by coolant compression can reduce the reliability of a sensor and wiring of the position detecting sensor inside the BLDC motor complicates the manufacturing process and increases the size of the motor.
Solutions exist whereby a xe2x80x9cself-commutatingxe2x80x9d or xe2x80x9cself-controlledxe2x80x9d mode of operation of the motor is used. By monitoring the BEMF generated in the windings, and more particularly, the zero crossing points of such back electromotive force (BEMF), the position of the rotor at a particular time is determined. BEMF is the voltage induced on a winding by a changing magnetic field present in the motor. The movement of a rotor pole contributes to the changes in the magnetic field (due to the magnetic field in the rotor), and therefore the BEMF provides some information about the instantaneous position of the rotor. A change in the sign of the BEMP occurs when a rotor pole passes through the center of the floating armature coil. By detecting zero-crossings in the BEMF, adequate information about rotor positions is provided.
The concise topology of the inverter fed 3-phase star-shaped BLDCM is shown in FIG. 1. In such a BLDC motor operated in the self-controlled mode, commutations occur at the instants when two of the three stator phases have equal BEMF to obtain the optimal efficiency of the motor. These commutation instants are shown clearly in FIG. 2 as t0, t1, t2, t3, t4 and t5. It is seen that such commutation events occur at 30xc2x0 delay from the corresponding zero-crossing points of BEMF waveforms. Therefore, commutation instants are generated from delaying the detected BEMF zero-crossing points for about a value of (30xc2x0+k*60xc2x0), wherein k=0, 1, 2 . . .
The BEMF is a terminal voltage of a winding when the winding is not electrically driven by the external driving circuit (as the so-called floating, high impedance or tri-state mode). By differentially monitoring the voltage across the floating phase, the point at which the voltage is zero, or xe2x80x9czero crossingxe2x80x9d is established. With this information, commutation (switch to the next winding phase) of a specific angle, normally 30 or 90 electrical degrees after the zero crossing is performed using a timer (either analog or digital).
There is inevitably a large voltage transition during the commutation due to a flyback current of the motor windings. An example is the waveform of the terminal voltage of phase A according to an analytical equivalent circuit of a symmetric VLCD motor, as shown in FIG. 3 . At the beginning of period t0-t1 when Qaxe2x88x92 is turned off and phase A is in a floating state. However, due to the presence of inductive element, ia still continues to flow through Da+, which is the so-called flyback current. Before a decreased to zero, there""s a short interval when all three phases are conducting current, and Va equals to Udc. Such a short interval is called commutation interval and results in a flyback pulse on terminal voltage. These flyback pulses also make transitions through zero and could cause erroneous indications of a zero crossing.
Furthermore, when the inverter is operating in PWM mode, another problem will yet arise. For mains powered motors, such as those commonly used to power domestic appliances, the DC power supply Vs is directly derived from the AC mains and may thus have a value of around +300V relative to the ground voltage for a 220V mains AC supply. The voltage actually supplied to each of the windings of the motor is controlled by pulse width modulation (PWM) of the DC supply voltage. In PWM the duration of each pulse is adjusted to provide the desired average voltage and current level. Meanwhile, in motor control systems generally, the control logic gets one or more data inputs to determine the commanded velocity of the motor, and accordingly controls PWM duty of the inverter to apply the correct average drive voltage to the motor so as to regulating the motor velocity as desired.
Various PWM schemes are possible, depending on which of the six switches in the inverter is in chopping mode. There are also two ways of handling the PWM switching: 4-quadrant chopping and 2-quadrant chopping. In 2-quadrant chopping, only one of the two simultaneous-conducting switches is in PWM chopping. Taking the 2-quadrant pattern with high-side or low-side chopping pattern for example. In the high-side (low-side) chopping mode, the low (high) side switch is kept ON during the 120 degree interval and the high (low) side switches switch according to the pulsed signal. In the 4-quadrant chopping technique both of two switches corresponding to the two phases in ON mode are driven by the same signal: the two switches are turned-on and turned-off at the same time.
These PWM chopping make multiple transitions through zero and could cause erroneous indications of pseudo zero crossings. The detailed waveform of terminal voltage of the inverter in high-side chopping mode, including the commutation interval and PWM chopping effect, is shown in FIG. 5. It can be seen that additional method should be applied to filter out the extra pseudo zero crossings.
Several methods have therefore been proposed to obtain more accurate zero-crossing detection:
1) Converting the BEMF to linear driving before the time of expected detection; and
2) Sampling the BEMF synchronously with the PWM switching.
Method 1 of converting the BEMF to linear driving before the time of expected detection has the drawback that the linear loop will have a long settling time and also an extra current ripple. Method 2 of sampling the BEMF synchronously with the PWM switching has the drawback of jitter in detection and complicated hardware switches and/or software tasks are demanded.
There are many cases that PWM chopping frequency is not high enough to meet the fundamental frequency of stator current fed into the BLDC motor, which lie in BLDC motor systems of high power rate such as power domestic appliances like air conditioner compressor. Wherein the PWM chopping frequency commonly ranges from 3 kHz to 10 kHz, while the armature current is of a fundamental frequency range up to 200 Hz and of a commutating frequency up to 1.2 khz.
In such situations, method 1 mentioned above is not suitable because of extremely large current ripple. Once method 2 is adopted, unless interpolation of the sampled BEMF values is applied, false detection of BEMF ZCP will be generated because the zero crossing events may occur during PWM off periods. The error between the false detection and the correction is at most equals to an off period of PWM. Due to the proximity of PWM frequency and the commutation frequency, such an error is not tolerable. While the necessity of interpolation will further complicate the controller apparatus, method 2 unlikely a probable solution.
It is an object of the invention to provide a control system, which operates conveniently with a PWM speed regulation of a sensorless BLDC motor with a plurality of windings. A hybrid method is present in the invention, which achieves reliable sensorless rotor position detection with relative concise and low cost circuit as well as little software expense.
In the provided method, BEMF induced in stator windings is utilized to indicate the commutation instant. This is done by phase-shifting the zero crossing point of BEMF 90 electric degree with a filter section and software compensation. Meanwhile, the falling edge detection method is applied to override PWM chopping and commutation noise.
According to principles of the present invention, a control apparatus is provided for a multiple-phase BLDC motor in which one phase is set at high impedance for measuring the back electromotive-force of the motor while one other phase is supplied by PWM signal. The control device includes a comparator for generating the BEMF zero crossing points (ZCP), and means for deriving rotor position information by detecting the falling edge (or rising edge for low-side chopping case) of the generated ZCPs.